The use of Polidori's plasticity and activity charts in classifying some residual lateritic soils from Nigeria

Over the years, Casagrande plasticity chart is mainly used to classify fine grain soils. However, the use of the plasticity chart has been questioned recently and this has led to the development of a new plasticity chart. Polidori in 2007 and 2009, respectively, developed the new plasticity and activity charts using the Atterberg's limits of pure clays (montmorillonite and kaolinite clay minerals) and their mixture with fine silica sand in different proportions. The applicability of Polidori's charts was evaluated using some residual lateritic soils from Nigeria. On the Casagrande's plasticity chart, the lateritic soils mostly plot above the A-line in the zone designated as clay and classified as either CL or CH. However, on the Polidori's plasticity chart, the lateritic soils classified as CL or CH, whereas on Casagrande's plasticity chart they are classified as ML or MH and vice versa. The classifications obtained from Polidori's plasticity chart are predominantly in agreement with the main soil fractions or component of the soils. This is different from the classification obtained from Casagrande's plasticity chart where lateritic soils with lower clay fractions than their silt/sand fractions are classified as clayey soils. Polidori's activity chart shows that lateritic soils that lie in the same plastic zone may show different behavior due to the different properties of the clay minerals in the soils. In cases where the lateritic soils lie in the zone that is not corresponding to their clay contents on the Polidori's plasticity chart, we presume that other factors apart from those stated by Polidori might also be responsible. Although the use of Polidori's plasticity chart gives a fair classification of the lateritic soils, nevertheless the peculiarity of residual soils such as the in situ structure that influenced the properties of the soils and properties developed due to weathering effects must be taken into consideration as well.